Size distribution dynamics reveal particle-phase chemistry in organic aerosol formation

Abstract

Organic aerosols are ubiquitous in the atmosphere and play a central role in climate, air quality, and public health. The aerosol size distribution is key in determining its optical properties and cloud condensation nucleus activity. The dominant portion of organic aerosol is formed through gas-phase oxidation of volatile organic compounds, so-called secondary organic aerosols (SOAs). Typical experimental measurements of SOA formation include total SOA mass and atomic oxygen-to-carbon ratio. These measurements, alone, are generally insufficient to reveal the extent to which condensed-phase reactions occur in conjunction with the multigeneration gas-phase photooxidation. Combining laboratory chamber experiments and kinetic gas-particle modeling for the dodecane SOA system, here we show that the presence of particlephase chemistry is reflected in the evolution of the SOA size distribution as well as its mass concentration. Particle-phase reactions are predicted to occur mainly at the particle surface, and the reaction products contribute more than half of the SOA mass. Chamber photooxidation with a midexperiment aldehyde injection confirms that heterogeneous reaction of aldehydes with organic hydroperoxides forming peroxyhemiacetals can lead to a large increase in SOA mass. Although experiments need to be conducted with other SOA precursor hydrocarbons, current results demonstrate coupling between particle-phase chemistry and size distribution dynamics in the formation of SOAs, thereby opening up an avenue for analysis of the SOA formation process.

Role of the reaction of stabilized Criegee intermediates with peroxy radicals in particle formation and growth in air

“…Accurate mass measurements (Table 2) of selected OLI-D oligomers agree well with the expected elemental composition for these peroxyhemiacetals. Formation of peroxyhemiacetals has also been suggested in SOA from ozonolysis of 1-tetradecene, 69 a-pinene, 16,23 and ethylene, 44 and photooxidation of dodecane 70 and toluene. 71 Fig .…”

Influence of vapor wall loss in laboratory chambers on yields of secondary organic aerosol

“…The small optimal α values required to reproduce the observed seed SA dependence likely reflect mass-transfer limitations within the particle phase, which can occur for highly viscous SOA particles (19). When α ∼10 −3 , mass accommodation is relatively slow and the vapors and particles cannot be assumed to be in instantaneous equilibrium.…”

Oxidation of a model alkane aerosol by OH radical: the emergent nature of reactive uptake

“…Comprehensive microscopic models that couple reaction and transport enable an internal view of the transformation of a particle with time as oxidation progresses. [24][25][26][27][28][29] This more detailed treatment has significantly broadened thinking about the complexity of aerosol transformations, but many of the required rate constants have not been measured. Additionally, diffusion is treated with a constant average rate initially determined from Fickian kinetics but not dynamically varied as local concentration gradients change throughout the oxidation process, leading to variances from the actual rates, and the oxidation reactions are simplified to include only general steps rather than the detailed free radical chains that are proposed to occur.…”